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Whenever the potential fathers of seeds differ in quality and whenever pollen is available in excess of the amount necessary to sire seeds, plants have both the opportunity and the selective pressure to mate nonrandomly (181). Plant mating may be nonrandom at several genetic, structural, and temporal levels. Genetically, mates may be sorted on the basis of relatedness to the seed parent (121, 137), complementarity of maternal and paternal genotypes (174), and the characters of pollen and pollen donors. Structurally, the physiological decisions that regulate mating may occur among the pollen grains and ovules within individual flowers and fruits (159, 181), among the fruits along branches (103), or across entire plants. Temporally, processes that produce nonrandom mating may occur both before and after pollen arrives on stigmas, during all of the steps from pollen germination through seed maturation, and …